Problem: There are $20$ ants eating some crumbs. $1$ was a red ant and the rest were black ants. How many were black ants?
Solution: We can solve this problem in two ways: $ \begin{aligned}&1 \text{ red ant}\\ +&\underline{ {?} \text{ black ants}}\\ =&{20} \text{ ants in total}\end{aligned}$ Or: $ \begin{aligned}&{20} \text{ ants in total}\\ -&\underline{ 1 \text{ red ant}}\\ =&{?} \text{ black ants}\end{aligned}$ ${19}$ black ants